Monte Carlo modeling and optimization of contrast-enhanced radiotherapy of brain tumors.

Contrast-enhanced radiotherapy involves the use of a kilovoltage x-ray beam to impart a tumoricidal dose to a target into which a radiological contrast agent has previously been loaded in order to increase the x-ray absorption efficiency. In this treatment modality the selection of the proper x-ray spectrum is important since at the energy range of interest the penetration ability of the x-ray beam is limited. For the treatment of brain tumors, the situation is further complicated by the presence of the skull, which also absorbs kilovoltage x-ray in a very efficient manner. In this work, using Monte Carlo simulation, a realistic patient model and the Cimmino algorithm, several irradiation techniques and x-ray spectra are evaluated for two possible clinical scenarios with respect to the location of the target, these being a tumor located at the center of the head and at a position close to the surface of the head. It will be shown that x-ray spectra, such as those produced by a conventional x-ray generator, are capable of producing absorbed dose distributions with excellent uniformity in the target as well as dose differential of at least 20% of the prescribed tumor dose between this and the surrounding brain tissue, when the tumor is located at the center of the head. However, for tumors with a lateral displacement from the center and close to the skull, while the absorbed dose distribution in the target is also quite uniform and the dose to the surrounding brain tissue is within an acceptable range, hot spots in the skull arise which are above what is considered a safe limit. A comparison with previously reported results using mono-energetic x-ray beams such as those produced by a radiation synchrotron is also presented and it is shown that the absorbed dose distributions rendered by this type of beam are very similar to those obtained with a conventional x-ray beam.